A vehicle seat belt retractor includes belt webbing wound on a spool of the retractor. A spool locking mechanism blocks withdrawal of seat belt webbing from the spool to help protect a vehicle occupant in the event of a vehicle collision. In some seat belt retractors, the spool locking mechanism is electrically actuated. To actuate the spool locking mechanism, a sensor senses a condition indicative of a vehicle collision and a controller sends an electrical signal to the retractor. In one known retractor, the electrical signal actuates an electromagnet which causes a pawl to move into engagement with a ratchet wheel on the retractor. In another known retractor, the actuation of an electromagnet creates a magnetic attractive force to restrain rotation of a magnetic disc and thereby to actuate the spool locking mechanism of the retractor.
U.S. Pat. Nos. 4,363,991, 5,206,557 and 5,642,015 disclose micro electro-mechanical transducers. These transducers are small devices which are capable of transforming a small electric current into mechanical motion.